The ability to transmit light through thin, flexible rods has many useful applications. In the visible light region of the optical light spectrum, this has been accomplished by low cost glass filament or plastic fibers. In the mid infra-red (IR) region of the spectrum, however, suitable fibers are difficult, if not impossible, to fabricate. Many workers have, therefore, focused their attention on hollow light guides, generally known as lightpipes.
The lightpipes should be flexible and ideally have a transmittivity greater than 90% per meter. Although, for some applications, the lightpipes may be as short as 1 inch, they should be manufacturable in lengths which exceed 1 meter and with an inner diameter of about 1 millimeter, and should be inexpensive, non-toxic, and easily manufactured. Lightpipes having such properties are suitable for numerous military, scientific, medical, and industrial applications.
A variety of approaches to the problem of developing such lightpipes have been taken, some meeting with limited success. There are several existing solid-type lightpipes relying on total internal reflection, but none of these has yet reached commercial realization. In the past, the methods of fabrication were capable of producing only relatively short lengths per manufacturing cycle, thus resulting in high cost per unit length. The hollow waveguide described by Katherine D. Laakmann in the U.S. Pat. No. 4,652,083 is available commercially, but suffers from limited flexibility and length. It is also more difficult than desired to manufacture because of the difficulty in depositing one or more thin film layers of precise thickness onto the interior wall of a long narrow lightpipe which may have an inside diameter of 1.0 to 1.5 millimeter.
In order for a hollow tube of approximately one meter length to transfer useful amounts of mid-IR radiation, it is necessary that the inner wall of the tube be coated with a reflection-enhancing material. This coating consists of one or more thin dielectric layers sometimes in combination with a thin layer of metal undercoating. Lightpipes having such coatings have been described in co-pending U.S. patent application Ser. No. 139,285, now U.S. Pat. No. 4,805,987 by Katherine D. Laakmann and Peter Laakmann, as well as U.S. Pat. Nos. 4,652,083, 4,688,892, and 4,688,893.
None of these coating methods, however, is conducive to depositing a coating directly on the inside of a long, narrow tube. It is therefore desirable to have an economical method of manufacturing hollow lightpipes having reflection-enhancing materials coated to their inner surfaces. In order to manufacture a low cost hollow lightpipe, it will be necessary to manufacture it via a continuous, linear process capable of producing hundreds of meters at a time. The resulting lightpipe should have the desirable characteristics noted above.